This invention relates to ultrasound imaging systems and, more particularly, to methods for ultrasound imaging using coded excitation.
Conventional ultrasound imaging systems comprise an array of ultrasonic transducer elements which transmit an ultrasound beam and receive the reflected beam from the object being studied. Such operation comprises a series of measurements in which the focused ultrasonic wave is transmitted, the system switches to receive mode after a short time interval, and the reflected ultrasonic wave is received, beamformed and processed for display. Typically, transmission and reception are focused in the same direction during each measurement to acquire data from a series of points along an acoustic beam or scan line. The receiver is dynamically focused at a succession of ranges along the scan line as the reflected ultrasonic waves are received.
For ultrasound imaging, the array typically has a multiplicity of transducer elements arranged in one or more rows and driven with separate voltages. By selecting the time delay (or phase) and amplitude of the applied voltages, the individual transducer elements in a given row can be controlled to produce ultrasonic waves which combine to form a net ultrasonic wave that travels along a preferred vector direction and is focused at a selected point along the beam. The beamforming parameters of each of the firings may be varied to provide a change in maximum focus or otherwise change the content of the received data for each firing, e.g., by transmitting successive beams along the same scan line with the focal point of each beam being shifted relative to the focal point of the previous beam. For a steered array, by changing the time delays and amplitudes of the applied voltages, the beam with its focal point can be moved in a plane to scan the object. For a linear array, a focused beam directed normal to the array is scanned across the object by translating the aperture across the array from one firing to the next.
The same principles apply when the transducer probe is employed to receive the reflected sound in a receive mode. The voltages produced at the receiving transducer elements are summed so that the net signal is indicative of the ultrasound reflected from a single focal point in the object. As with the transmission mode, this focused reception of the ultrasonic energy is achieved by imparting separate time delay (and/or phase shifts) and gains to the signal from each receiving transducer element.
An ultrasound image is composed of multiple image scan lines. A single scan line (or small localized group of scan lines) is acquired by transmitting focused ultrasound energy at a point in the region of interest, and then receiving the reflected energy over time. The focused transmit energy is referred to as a transmit beam. During the time after transmit, one or more receive beamformers coherently sum the energy received by each channel, with dynamically changing phase rotation or time delays, to produce peak sensitivity along the desired scan lines at ranges proportional to the elapsed time. The resulting focused sensitivity pattern is referred to as a receive beam. Resolution of a scan line is a result of the directivity of the associated transmit and receive beam pair.
The output signals of the beamformer channels are coherently summed to form a respective pixel intensity value for each sample volume in the object region or volume of interest. These pixel intensity values are log-compressed, scan-converted and then displayed as an image of the anatomy being scanned.
Coded excitation is the transmission of long encoded pulse sequences and decoding of the received signals in order to improve image SNR (signal-to-noise ratio) and/or resolution. The energy contained in a long transmit pulse sequence is compressed into a short time interval on receive by virtue of the code. Coded excitation is a well-known technique in medical ultrasound imaging. For example, the use of Golay codes is disclosed in U.S. Pat. No. 5,984,869, issued Nov. 16, 1999 and assigned to the instant assignee. Because Golay codes use a pair of transmits for each transmit focal zone, only minimal tissue motion between the two transmits can be tolerated to avoid decoding errors and sidelobes (or rangelobes). As such, the two-transmit version of Golay-coded excitation may be unsuitable for imaging applications where fast tissue motion is present, such as imaging the heart.
Thus there is a need for a method of ultrasound imaging using Golay-coded excitation which increases the motion robustness of the system.
More than two focused and coded beams are transmitted at a fundamental frequency to each transmit focal zone or position during the acquisition of acoustic data for a single image frame. The resulting receive vectors then undergo receive correlation and xe2x80x9cslow-timexe2x80x9d filtering, i.e., filtering from firing to firing. The xe2x80x9cslow-timexe2x80x9d filtering is accomplished by multiplying each set of receive correlation filter coefficients by respective scalar weightings before summing the resulting set of filtered receive vectors for subsequent processing to form one image scan line. The employment of more than two firings serves to suppress the sidelobe response without altering the mainlobe response substantially in the presence of tissue motion after the weighted receive vectors have been summed.
Since frame rate is inversely proportional to the number of firings per focal zone, it is desirable to have as few firings as possible per focal zone, so the preferred embodiment has three firings: the first and third firings being pulse sequences encoded with the Golay code A and the second firing being a pulse sequence encoded with the Golay code B of a Golay code pair {A, B}. The echoes from the three firings are summed together after receive correlation, with the first and third echoes being weighted less than the second echo. Preferably the receive correlation filter coefficients are designed to pass a desired band of frequencies.
In accordance with a preferred embodiment, a [0.5, 1.0, 0.5] weighting is used on receive for summing the three echoes. The resulting effective wall filter for the mainlobe is [0.5, 1.0, 0.5], while that for the sidelobes is [xe2x88x920.5, 1.0, xe2x88x920.5]. The sidelobe response is suppressed by 5-20 dB below that for two-transmit Golay-coded excitation, while the mainlobe is reduced by less than 3 dB. Although substantial sidelobes still result at the higher tissue speeds, these are expected to be tolerable due the inherent visual blurring from very fast motion.